Conventionally, it is known that a positioning result from the GPS (Global Positioning System) positioning contains a bias error due to propagation delay, a positioning error due to multi-path, and the other random errors due to various factors.
The bias error occurs mainly due to a propagation delay when a GPS signal passes through an ionization layer between a GPS satellite and the earth. The bias error always occurs because the ionization layer mainly causes it. The degree of errors varies with ionization layer states every moment.
A conventional apparatus such as a car navigation system receives information about a bias error from an FM broadcast station (so-called DGPS station) in the form of broadcast waves. The apparatus corrects a position coordinate measured by a GPS receiver in accordance with the received bias error information (e.g., see Patent Document 1).                Patent Document 1: H10-2002-185600 A        
However, the FM receiver needs to be mounted on the apparatus for the method that receives the bias error information from the FM broadcast station and corrects the position coordinate measured by the GPS receiver. Accordingly, the product cost increases.
When a measuring point is distant from an FM broadcast station, that method causes a difference between a bias error at the measuring point and that in the FM broadcast station due to surrounding environment differences. Even though receiving the bias error information from the FM broadcast station, a reception destination apparatus (navigation system) cannot accurately remove the bias error from the positioning result from the GPS receiver. The apparatus cannot accurately detect the position coordinate for the current position. Further, the conventional method corrects a positioning result from the GPS receiver based on less accurate bias error information received from the FM broadcast station. The positioning accuracy may be degraded.